Nice move, Graf! I smell a conflict.
edit: strike all this below. I have a better idea. We'll do that "role-playing" thing and I'll set it up to get Tira in on things.
I'll allow you to make an "Aid Another" check to give Erias a +2 bonus to his next check. Your words impress on him the need for haste/you aid him in his ritual/whatever. If you fail, though, he'll take a -2 - your link to dead gods taints him/your constant nagging to hurry up bothers him/or however we want to describe that failure.
This Aid Another check won't apply a success or failure to the overall skill challenge, and you don't have to make it if you don't want to. The DC is the general DC for the skill challenge; it's a secret, but it's not 10. 
If Erias wants to perform a "Ritual", he can; the skill he uses will tell us how he does that. Diplomacy? Maybe he gets someone to cast for him. Religion? Maybe he uses the resources of the temple (whatever they might be, a relic, a magic item) to perform a Ritual. My point is that I won't veto a skill check that has him "cast a spell" that's not on his character sheet.
You head to the Court of Erathis, a stately building in the middle of town. It is hard to miss, even for newcomers like yourselves - it's just like any other Court dedicated to Erathis you've seen in the Wildlands settlements: white walls, made of stone quarried with no tools but the Word of Law
1; the All-Seeing Eye, who sees all transgressions, looming over the entrance; and the Two Blind Seers of Justice carved into the supporting columns, one with sword in hand, the other with scales. The familiar Covenant of Erathis is carved on its doors: "All are Equal Before Me."
It's well lit inside, slightly dusty, and full of that musty odour of old books and parchments. Clerks are writing at their desks; they pause, look up at you for a moment, and turn back to their work.
A priest approaches, a thin, balding, middle-aged man with ink-stained fingers. "Welcome," he says, "to the Court of Erathis in Bel Tungin. He has gifted us with the Rule of Law, that we may spread civilization to all who seek it.
"Do you have a greivance to file?" he asks, staring at Tira.
His eyes narrow, and he focuses on the arcane wand at her belt. "Tell me, miss - is that a wizard's wand? I would not have an Oathbreaker
2 defile His Court. It is against His Law."
1 - A Stone Shape spell, cast by acolytes simply reading the law to the stone.
2 - "Oathbreakers" are what clerics of Eriathis call warlocks; for breaking the Sacred Oath: Thou Shalt Not Trade in Souls. "Even your own" is assumed part of the Law, as written in the 1,811 Clarifications of Thondad the Elder.
Anyway, within the temple lies the "All-Seeing Eye" (of Justice!), and you can use that to attempt a scry. How do you want to gain access to it? You'll need to roll!
If you want to use a power or an attack, you can; you can use Eyebite to "mess with his mind", or Curse of the Dark Dream to really mess him up good, though neither one will kill him (just break his mind, that's all
). A hit I'll consider a success on the skill challenge, and you'll gain access to the All-Seeing Eye, and the other clerks won't think anything of it.